Surface preparation for electrolytic inorganic finishes

Abstract

The present disclosure relates to substrate surface treatment methods and compositions. In at least one aspect, a method includes degreasing a metal substrate; performing a first alkaline cleaning on the metal substrate; etching the metal substrate by electropolishing techniques or with an alkaline etchant solution to form an etched metal substrate, wherein the etched metal substrate comprises an average surface roughness (R a ) of about 8 μin (0.2 μm) to about 16 μin (0.4 μm); deoxidizing the etched metal substrate to form a processed metal substrate; performing alkaline cleaning on the processed metal substrate; optionally performing etch-cleaning and deoxidizing if needed to remove oxides and depositing a coating onto the processed metal substrate to form a coated metal substrate.

1 . A method comprising:
 degreasing a metal substrate;   performing a first alkaline cleaning on the metal substrate;   etching the metal substrate with an alkaline etchant solution to form an etched metal substrate, wherein the etched metal substrate comprises an average surface roughness (R a ) of about 250 μin (6.4 μm) or less;   deoxidizing the etched metal substrate to form a processed metal substrate;   optionally performing a second alkaline cleaning on the processed metal substrate; and   depositing a coating onto the processed metal substrate to form a coated metal substrate.   
     
     
         2 . The method of  claim 1 , wherein the alkaline etchant solution comprises about 2 vol % to about 6 vol % of triethanolamine, about 2 vol % to about 30 vol % of a sodium sulfide additive, about 10 vol % to about 40 vol %, about 75 g/L or less of dissolved metal content, and about 6 g/L or less of ethylenediaminetetraacidic acid (EDTA). 
     
     
         3 . The method of  claim 1 , wherein the processed metal substrate comprises an average surface roughness R a  of about 8 μin (0.2 μm) to about 16 μin (0.4 μm). 
     
     
         4 . The method of  claim 1 , wherein the sum of the highest peak and lowest valley (R t ) of the surface of the processed metal substrate is about 158 μin (4 μm) to about 236 μin (6 μm). 
     
     
         5 . The method of  claim 1 , wherein deoxidizing the etched metal substrate comprises immersing the etched metal substrate within a deoxidizing solution comprising about 40 vol % to about 60 vol % of nitric acid. 
     
     
         6 . The method of  claim 1 , wherein depositing a coating onto the processed metal substrate comprises electrolytically depositing an inorganic finish thereon, the electrolytic deposition of the inorganic finish comprising:
 attaching the processed metal substrate to a counter electrode,   electrically connecting the processed metal substrate to a power supply,   immersing the processed metal substrate within an inorganic finishing solution,   applying a current density to the processed metal substrate to form a coated metal substrate,   removing the coated metal substrate from the inorganic finishing solution, and   rinsing the coated metal substrate.   
     
     
         7 . The method of  claim 6 , wherein the current density is applied to the processed metal substrate for about 2 min to about 5 min. 
     
     
         8 . The method of  claim 1 , wherein the coating applied to the metal substrate provides corrosion protection in a salt fog environment of 336-hours minimum without pits having a diameter of about 0.031 in or less; fewer than 5-pits in 30-in 2  and fewer than 15-pits in an area of 150-in 2 . 
     
     
         9 . The method of  claim 6 , wherein the coating applied to the metal substrate provides corrosion protection in a salt fog environment of 336-hours minimum without pits having a diameter of about 0.031 in or less; fewer than 5-pits in 30-in 2  and fewer than 15-pits in an area of 150-in 2 . 
     
     
         10 . A method, comprising:
 degreasing a metal substrate;   performing a first alkaline cleaning on the metal substrate;   etching the metal substrate with an alkaline etchant solution to form an etched metal substrate, the alkaline etchant solution comprising about 2 vol % to about 6 vol % of triethanolamine, about 2 vol % to about 30 vol % of a sodium sulfide additive, about 10 vol % to about 40 vol %, about 75 g/L or less of dissolved metal content, and about 6 g/L or less of ethylenediaminetetraacidic acid (EDTA);   deoxidizing the etched metal substrate with a deoxidizing solution comprising about 40 vol % to about 60 vol % of nitric acid to form a processed metal substrate;   performing a second alkaline cleaning on the processed metal substrate; optionally performing alkaline etch cleaning with said alkaline etchant and deoxidizing solution comprising about 40 vol % to about 60 vol % of nitric acid, which may be used alone or in combination with iron based or chromated deoxidizing solutions to remove oxides if needed nitric acid deoxidizing solution alone or in combination with other deoxidizing solutions to remove oxides as needed; and   depositing a coating onto the deoxidized metal substrate to form a coated metal substrate.   
     
     
         11 . The method of  claim 10 , wherein depositing a coating onto the processed metal substrate comprises electrolytically depositing an inorganic finish thereon, the electrolytic deposition of the inorganic finish comprising:
 attaching the processed metal substrate to a counter electrode,   electrically connecting the processed metal substrate to a power supply,   immersing the processed metal substrate within an inorganic finish solution,   applying a current density to the processed metal substrate to form a coated metal substrate, removing the coated metal substrate from the inorganic finishing solution, and   rinsing the coated metal substrate.   
     
     
         12 . The method of  claim 10 , wherein the processed metal substrate comprises an R a  of about 8 μin (0.2 μm) to about 16 μin (0.4 μm). 
     
     
         13 . The method of  claim 10 , wherein the (R t ) of the surface of the processed metal substrate is about 158 μin (4 μm) to about 236 μin (6 μm). 
     
     
         14 . The method of  claim 11 , wherein the current density is applied to the processed metal substrate for about 2 minutes to about 5 minutes. 
     
     
         15 . The method of  claim 11 , wherein the coating applied to the metal substrate provides corrosion protection in a salt fog environment of 336-hours minimum without pits having a diameter of about 0.031 in or less, fewer than 5-pits in 30-in 2 , and fewer than 15-pits in an area of 150-in 2 . 
     
     
         16 . A method comprising:
 degreasing a metal substrate;   performing a first alkaline cleaning on the metal substrate;   etching the metal substrate with an alkaline etchant solution to form an etched metal substrate, the alkaline etchant solution comprising about 2 vol % to about 6 vol % of triethanolamine, about 2 vol % to about 30 vol % of a sodium sulfide additive, about 10 vol % to about 40 vol %, about 75 g/L or less of dissolved metal content, and about 6 g/L or less of ethylenediaminetetraacidic acid (EDTA);   deoxidizing the etched metal substrate with a deoxidizing solution to form a processed metal substrate, the deoxidizing solution comprising about 40 vol % to about 60 vol % of nitric acid;   performing a second alkaline cleaning on the processed metal substrate; optionally performing alkaline etch cleaning with said alkaline etchant and deoxidizing solution comprising about 40 vol % to about 60 vol % of nitric acid, which may be used alone or in combination with iron based or chromated deoxidizing solutions to remove oxides if needed;   depositing a coating onto the processed metal substrate to form a coated metal substrate, wherein depositing a coating onto the processed metal substrate comprises electrolytically depositing an inorganic finish thereon, the electrolytic deposition of the inorganic finish comprising:
 attaching the processed metal substrate to two or more counter electrodes, 
 electrically connecting the processed metal substrate to a power supply, 
 immersing the processed metal substrate within an inorganic finishing solution, 
 applying a current density to the processed metal substrate to form a coated metal substrate, and 
 rinsing the coated metal substrate. 
   
     
     
         17 . The method of  claim 16 , wherein the current density applied to the processed metal substrate is about 0.2 mA/cm 2  to about 0.5 mA/cm 2 . 
     
     
         18 . The method of  claim 16 , wherein the current density is applied to the processed metal substrate for about 2 minutes to about 5 minutes. 
     
     
         19 . The method of  claim 16 , wherein the current density is applied to the processed metal substrate as a constant current. 
     
     
         20 . A coated metal substrate, comprising:
 an etched metal substrate, the etched metal substrate comprising a metal substrate having an average surface roughness of about 79 μin (2 μm) to about 8 μin (0.2 μm), wherein:
 the average surface roughness of the metal substrate is the result of etching the metal substrate in an etching solution and deoxidizing the metal substrate in a deoxidation solution comprising about 40 vol % to about 60 vol % of nitric acid, and 
   an inorganic finish disposed over the etched metal substrate, wherein the conversion coating comprises a morphology substantially as shown in  FIG.  2   .